This invention is generally in the field of Nuclear Magnetic Resonance (NMR) based techniques, and relates to a device and method for magnetic resonance imaging (MRI). Although not limited thereto, the invention is particularly useful for medical purposes, to acquire images of cavities in a human body, but may also be used in any industrial application.
MRI is a known imaging technique, used especially in cases where soft tissues are to be differentiated. Alternative techniques, such as ultrasound or X-ray based techniques, which mostly utilize spatial variations in material density, have inherently limited capabilities in differentiating soft tissues.
NMR is a term used to describe the physical phenomenon in which nuclei when placed in a static magnetic field, respond to a superimposed alternating (RF) magnetic field. It is known that when the RF magnetic field has a component perpendicular in direction to the static magnetic field, and when this component oscillates at a frequency known as the resonance frequency of the nuclei, then the nuclei can be excited by the RF magnetic field. This excitation is manifested in the temporal behavior of nuclear magnetization following the excitation phase, which in turn can be detected by a reception coil and termed the NMR signal. A key element in the utilization of NMR for imaging purposes is that the resonance frequency, known as the Larmor frequency has a linear dependence on the intensity of the static magnetic field in which the nuclei reside. By applying a static magnetic field, of which the intensity is spatially dependent, it is possible to differentiate signals received from nuclei residing in different magnetic field intensities, and therefore in different spatial locations. The techniques, which utilize NMR phenomena for obtaining spatial distribution images of nuclei and nuclear characteristics, are termed MRI.
In conventional MRI techniques, spatial resolution is achieved by superimposing a stationary magnetic field gradient on a static homogeneous magnetic field. By using a series of excitations and signal receptions under various gradient orientations a complete image of nuclear distribution can be obtained. Furthermore, it is a unique quality of MRI that the spatial distribution of chemical and physical characteristics of materials, such as biological tissue, can be enhanced and contrasted in many different manners by varying the excitation scheme, known as the MRI sequence, and by using an appropriate processing method.
The commercial application of MRI techniques suffers from the following two basic drawbacks: the expenses involved with purchasing and operating an MRI setup; and the relatively low signal sensitivity which requires long image acquisition time. Both of these drawbacks are linked to the requirement, in standard MRI techniques, to image relatively large volumes, such as the human body. This necessitates producing a highly homogeneous magnetic field and field gradients over the entire imaged volume, leading to extensive equipment size. Additionally, the unavoidable distance between a signal receiving coil and most of the imaging volume significantly reduces imaging sensitivity.
In order to better understand the inter-relation between hardware limitations and the characteristics of the obtained image, the typical MRI system components are described below in more detail. Such a system must include at least the following four components; (1) a strong static (DC) magnetic field source creating the primary static substantially homogeneous magnetic field in the entire volume to be imaged; (2) a transmission antenna (coil) and a transmission channel for transmitting No excitation pulses; (3) a reception antenna (coil) and a receiving channel for receiving the so-excited NMR signal; and (4) a magnetic field gradient source to spatially encode-the signals originating from the imaged volume.
The resolution of an image depends on many parameters, some of which are related to the imaging hardware, and some relate to the imaging technique or pulse-sequence used. Practically, however, the resolution is generally governed by two parameters: gradient field strength and signal to noise ratio (SNR) per volume cell (voxel).
It is known and disclosed for example in xe2x80x9cPrinciples of Nuclear Magnetic Resonance Microscopyxe2x80x9d, P. T. Callaghan, Oxford Science Publications, 1995, that in a noise-free (ideal) setting for 2-D Fourier spin-echo imaging, if the required resolution in the gradient dimension is xcex94x, then the resolution requirement can be written as follows:
xcex3xc2x7Gmaxxc2x7xcex94xxc2x7Tgrad≈xcfx80
wherein xcex3 is the gyromagnetic ratio, Gmax is the maximum achievable gradient (in Tesla/m) and Tgrad is the gradient pulse length. For typical gradient pulse lengths of about 1 msec (limited by signal decay, etc.), the required Gmax for a resolution of about 0.5xc3x970.5 mm is about 2-3 Gauss/cm. It tuns out that these high gradient values are hard to achieve over large volumes (typically 50xc3x9750xc3x9750 cm), especially since large gradient coils having large inductance values are reluctant to develop large currents over short periods of time. Moreover, creating high field gradients over large portions of the patient""s body can induce discomforting and even dangerous nerve activation, let alone unbearable acoustic noise during gradient transmission coming from the MRI machine itself.
The other factor that governs the resolution limit is the SNR per voxel, which is strongly related to reception coil sensitivity. Conventional, MRI machines have the reception antennas (coils) installed in the main body of the machine, and thus geometrically far from internal organs, which need to be imaged. This problem has been addressed in the past and partially solved by some remarkable innovations, disclosed for example in U.S. Pat. Nos; 5,699,801; 5,476,095; 5,365,928; 5,307814 and 5,050,607. Generally, these innovations consist of using an application-specific reception coil to be located in the vicinity of the tissue to be imaged (sometimes external and sometimes internal to the body), thus increasing receiving sensitivity, SNR and, eventually, the image resolution.
U.S. Pat. No. 5,572,132 discloses a concept of combining the static magnetic field source with the RF coil in a self contained intra-cavity medical imaging probe. Here, several permanent magnet configurations are proposed for creating a homogeneous magnetic field region external to the imaging probe itself, and several RF and gradient coil configurations that may be integrated in the imaging probe in order to allow autonomous imaging capabilities. The limitations of the autonomous probe of U.S. Pat. No. 5,572,132 are the fact that the imaging technique still requires a region of substantially homogeneous field, which, unfortunately, can be created in a very limited volume externally to the probe itself limiting a field of view (FOV) of the device, and the fact that the static field values created in this very limited region are substantially low. This limitation stems mostly from the fact that the static field sources can create sufficiently strong static magnetic field in a listed region around them. Further away from the magnets the static field strength drops significantly, up to a point where there is no sufficient SNR per voxel, and therefore no imaging feasibility. These limitations make it practically impossible to use the device for imaging, as opposed to measurement, purposes.
The use of a portable receiver coil in conjunction with external MRI machines (as disclosed in the above-mentioned U.S. Pat. Nos. 5,699,801; 5,476,095; 5,365,928; 5,307814 and 5,050,607) is not subject to this limitation, because the external field sources are capable of creating very strong magnetic field (typically 0.5 to 4 Tesla in medical imaging) over very large volumes (again: typically 50xc3x9750xc3x9750 cm""s). Although the receiver sensitivity xe2x80x9cbarrierxe2x80x9d is lifted by using an internal coil, other problems, such as difficulty and limitations in producing strong gradient fields over large volumes, together with the high cost of the external MRI setup, still hinder the application of this concept from becoming straightforward. In addition, although the SNR limitation is practically overcome by the portable coil systems, the gradient intensity limitation (due to either hardware difficulties or patient discomfort) still remains.
The conventional MRI setup typically utilizes expensive and complicated hardware means for creating a substantially homogeneous static magnetic field. The operation with such homogeneous static magnetic field allows operating in a narrow frequency bandwidth (typically a few Hz), which results in low noise or, rather, high signal to noise ratio (SNR) per spin-echo; the duration of which can be a few milliseconds. In this conventional scheme a relatively small number of such spin-echo signals (typically a few such signals) can be acquired during one excitation series, the duration of which is roughly limited by a typical spin transverse relaxation tire (known as T2).
There is accordingly a need in the art to facilitate high-resolution magnetic resonance imaging by providing a novel imaging device and method, as well as an MRI system utilizing the same, enabling to overcome external gradient field limitations. The present invention provides for combining the simplicity and low cost of a totally autonomous probe, with the ability to image larger volumes around the probe, without compromising high-resolution.
The present invention takes an advantage of the ability to operate in a substantially non-homogeneous static magnetic field, as long as the wideband reception and transmission channels are used, as well as specifically designed excitation pulse sequences. These provisions make it possible to achieve high SNR, which is similar to that achievable by the conventional MRI technique, by accumulating a large number of wide-bandwidth short duration echoes, in a similar overall acquisition time. This concept and devices utilizing the same are disclosed in co-pending applications assigned to the assignee of the present application. Generally speaking, according to this concept, the non-homogeneity of the static field created externally to the probe is used, rather than avoided, as a means for creating a gradient field inherently superimposed on a static field. The conventional spin-echoes excitation scheme is thus based on the use of a substantially non-homogeneous static field patterns which, on the one hand, increases the bandwidth, but, on the other hand, allows the accumulation of a large number of spin-echoes for averaging and SNR improvement. The scheme therefore allows substantially strong field gradients to be operated without losing the overall SNR, creating optimal conditions for high image resolution.
The main idea of the present invention is based on using a novel magnetic assembly (that may be a separate element or a part of an autonomous imaging device (probe)), in conjunction with external static field sources typically used in MRI systems. This magnetic assembly is of a kind that, when being accommodated within a primary static reasonably homogeneous magnetic field, it deforms the static magnetic field pattern to cause the creation of a region of substantial non-homogeneity of the static magnetic field in the vicinity of the magnetic assembly. Thus, the magnetic assembly creates a local static field gradient around the magnetic assembly, and, by moving the magnetic assembly within the entire static field region created by the external source, a FOV of the entire MRI system is moved from region to region.
The term xe2x80x9creasonably homogeneous magnetic fieldxe2x80x9d used herein signifies a field, whose original nonhomogeneity (prior to being affected by the magnetic assembly) in the intended vicinity of the assembly is negligible as compared to the non-homogeneity of the magnetic field as created by this assembly.
It is important to note that the magnetic assembly according to the invention deforms the static magnetic field pattern created by an external magnetic field source without creation of any additional magnetic field. In other words, the magnetic assembly is a passive element that affects (deforms) the static magnetic field pattern.
As indicated above, the magnetic assembly may be a part of the imaging probe, in which case transmission, reception and gradient coils are preferably integrated in the probe itself, while the primary static field is produced by external sources.
The imaging device according to the invention is a partially autonomous MRI probe, which can be applied to local imaging of an extensive region located externally to the probe in a static reasonably homogeneous magnetic field. The probe in such application is preferably integrated in a surface exploring or a minimally invasive device such as a catheter, which is connected to an external imaging console. The device according to the invention comprises a novel magnetic assembly, and preferably integrates all components necessary to allow magnetic resonance measurements and imaging of local surroundings of the magnetic assembly (i.e., field sources and antennas), except for an external source creating a reasonably homogeneous static magnetic field in the imaging volume. The device can be used for surface imaging or internal body imaging by insertion to the body using a catheter.
An MRI system according to the invention utilizes a primary static magnetic field source of a kind used in the conventional MRI systems (only with more lenient restrictions on field homogeneity), and an imaging device (probe) incorporating or preferably associated with other electromagnetic assemblies of the system.
Since a distortion of the external magnetic field created by the magnetic assembly is made in a well-defined manner, i.e., the resulting field pattern enables spatial mapping of each component of the received NMR signal, then it can be used as a local gradient field added to the otherwise homogeneous external field. The distortion can be made very intense, meaning a substantially high field gradient, but also very local, meaning over a relatively small volume (compared to the entire homogeneous field region) in probity to the distortion assembly. This way, the limitations associated with the generation of field gradients over larger volumes are overcome.
To create a basic local gradient of tie static magnetic field (i.e., separation in first dimension), the imaging probe may comprise a ferromagnetic material of a specific geometry creating constant distortion, or a special gradient coil may be located at the imaging probe itself. An additional gradient creating separation in the second dimension is preferably implemented by a gradient coil, since the gradient must be time varying. Additional gradient coils may be added to create separation in the third dimension
The present invention, therefore, provides a means for obtaining substantially high resolution MR images of a substantially extensive region in proximity to a magnetic field deforming assembly, while greatly reducing the homogeneity requirements for the externally produced static field, therefore greatly to reducing its cost.
An imaging method according to the invention is based on the use of a substantially non-homogeneous static magnetic field (created by the local probe-produced distortion in the static magnetic field) and on the use of wideband reception and transmission channels and pulse sequences specifically designed to make use of the substantially non-homogeneous static magnetic field.
There is thus provided according to one aspect of the present invention, a magnetic assembly for use in an MRI system, the magnetic assembly being of a kind which, when accommodated within a primary static reasonably homogeneous magnetic field, affects the static magnetic field to cause creation of a region of substantial non-homogeneity of the static magnetic field in a medium in the vicinity of the magnetic assembly, thereby creating a local static field gradient, which can be used for imaging.
The magnetic assembly may be in the form of a rod or sphere made of a ferromagnetic material. The region of non-homogeneous static magnetic field is in the form of a slice surrounding the magnetic assembly.
According to another aspect of the present invention, there is provided an MRI system comprising a first magnetic field source creating a primary static reasonably homogeneous magnetic field in a medium, a second magnetic field source creating a time-varying magnetic field in a region of said primary static reasonably homogeneous magnetic field to excite nuclei in the medium to generate NMR signals, and a receiver for receiving the NMR signals and generating data indicative thereof, wherein said MRI system comprises a magnetic assembly which, when being accommodated within said region of primary static reasonably homogeneous magnetic field, affects the static magnetic field pattern to cause creation of a region of substantial non-homogeneity of the static magnetic field in the vicinity of the magnetic assembly, thereby creating a local static field gradient, a field of view of the MRI system being defined by a region of the local static field gradient.
According to yet another aspect of the present invention, there is provided an imaging device for use with an external magnetic field source that creates a primary static substantially homogeneous magnetic field, the device comprising:
a magnetic assembly of a kind which, when accommodated within said primary static reasonably homogeneous magnetic field, affects the static magnetic field to cause creation of a region of substantial non-homogeneity of the static magnetic field in a medium in the vicinity of the magnetic assembly;
a magnetic field source for creating a time-varying magnetic field, which, when being applied to said region, is capable of exciting nuclei in at least a part of said region to generate NMR signals; and
a receiver for receiving the NMR signals and generating data indicative thereof.
The time-varying magnetic field source is in the form of at least one radio-frequency (RF) transmitting coil. The transmitting coil may be wound onto a rod-like magnetic assembly, in which case it excites nuclei within a circumferential region of the non-homogeneous static magnetic field (i.e., slice). Alternatively, the transmitting coil may be mounted adjacent to the rod-like magnetic assembly, such that the coil extends along an axis perpendicular to the longitudinal axis of the rod. In this case, the coil excites nuclei in a sector of the slice-like region of non-homogeneous static magnetic field.
The data generated by the receiver is received and processed in a control unit having suitable electronics and data processing means. The electronics generally comprises the following utilities:
1. A receiver channel, which includes tuning/matching components and several amplification stages of the received NMR signal, until it is finally digitized in the imaging console;
2. A transmission channel, which includes a D/A, a high-power amplifier and tuning/matching components to the transmission coil;
3. A gradient generator unit for generating gradient field pulses of the time-varying magnetic field at the required levels; and preferably
4. A motor unit for rotating the imaging probe about its longitudinal axis in order to create two-dimensional images, and, preferably, also for moving the probe along the longitudinal axis thereof in order to obtain three-dimensional images, for example, of segments of a blood vessel wall.
As indicated above, the imaging device according to the invention may be a fully autonomous probe device incorporating all necessary electromagnetic field and field-distortion sources, as well as a receiver coil, except for the static magnetic field source, which is produced externally. The autonomous probe device is further connected to an external imaging console, which together form a complete imaging system, integrating all the above electronic components.
The external imaging console (control unit) preferably comprises a personal computer (PC) having appropriate hardware operated by software for producing analog transmission signals, recording digitally the received spin-echo signals, processing the signals into images and displaying the obtained images on the PC monitor.
Thus, according to yet another aspect of the present invention, there is provided an MRI system comprising an external magnetic field source that creates a primary static substantially homogeneous magnetic field, an imaging probe device, and a control unit for receiving and analyzing data generated by the probe device to create images of a region of interest, wherein said imaging probe device comprises:
a probe portion having a magnetic assembly of a kind which, when accommodated within said primary static reasonably homogeneous magnetic field, affects the static magnetic field to cause creation of a region of substantial non-homogeneity of the static magnetic field in a medium in the vicinity of the probe portion;
a magnetic field source for creating a time-varying magnetic field, which, when being applied to said region, is capable of exciting nuclei in at least a part of said region to generate NMR signals; and
a receiver for receiving the NMR signals and generating data indicative thereof.
According to yet another aspect of the invention, there is provided a method for magnetic resonance imaging of a region of interest in a medium, said region of interest lying within a region a primary, static, reasonably homogeneous magnetic field, the method comprising the steps of:
(i) affecting said primary, static, reasonably homogeneous magnetic field with a magnetic assembly, so as to create a region of substantial non-homogeneity of the static magnetic field in the region of interest;
(ii) applying a time-varying magnetic field to at least a part of said region of interest to excite nuclei therein for generating NMR signals;
(iii) receiving the NMR signals and generating data indicative thereof,
(iv) analyzing the generated data to obtain an image of said region of interest.
Preferably, the time-varying magnetic field is capable of efficiently and simultaneously exciting nuclei characterized by a frequency bandwidth of more than 1% of a mean value of a magnetic resonance frequency for the nuclei.
The time-varying magnetic field created by the second magnetic field source has sufficiently high intensity and sufficiently wide frequency band, such that it is capable of efficiently and simultaneously exciting nuclear spins to generate NMR signals characterized by NMR frequency bandwidth of more than 1% (generally, 5%-200%, typically about 10%) of their mean frequency value. It should be understood that the term xe2x80x9csimultaneous excitationxe2x80x9d signifies excitation by a single pulse. Accordingly, the receiving of NMR signals is carried out with similar sufficiently high sensitivity and sufficiently wide frequency band.
The technique of the present invention thus consists of the following. A sample or body (or body part) to be imaged is placed in the static external magnetic field. The imaging device is connected to the external imaging console, and at least a probe portion of the device having the magnetic assembly that affects (deforms) the static magnetic field is brought to the vicinity of the ROI, by any known procedure, such as catheterization, surface scan, etc. When the static field-affecting magnetic assembly is located adjacent to the ROI, an operator initiates the imaging process via the imaging console controls. Imaging of the ROI is made possible in the following way:
The magnetic assembly affecting the static magnetic field actually deforms the magnetic field lines of the reasonably homogeneous static magnetic field, thereby creating a static non-homogeneous magnetic held region of a monotonic profile of sufficient absolute and gradient strength within the region of interest (ROI), which is external to the probe portion, and is in the form of a slice surrounding the probe portion. The exact geometry of the deformation depends on the geometry and properties of both external static field and magnetic assembly, and on their relative position and orientation thereof.
The time-varying magnetic field source (e.g., a transmission coil) is used to excite the nuclei located in the ROI, while the receiver (e.g., a coil) is used to receive the NMR signal (typically spin-echo) produced by the nuclei.
In order to produce a two-dimensional image of the ROI, two magnetic field gradients are used: The first gradient is inherent in the deformed static magnetic field, which generally changes substantially when moving radially away from the edge of the imaging probe (magnetic assembly). The second gradient is time-dependent and is generally directed perpendicular to the radial gradient i.e., the angle direction.
The time-varying gradient can be produced by any known suitable technique. For example, transmission coil designs disclosed in co-pending applications assigned to the assignee of the present application.
In order to produce a two-dimensional image of the ROI, a sequence of excitation pulses, preferably a multiple spin-echo sequence, such as the well-known CPMG, is transmitted by the transmission coil. This sequence has the advantage that a large number of spin-echoes can be recorded during a time frame of signal coherence, even at substantial static and RF field gradients.
Excitation of the ROI is preferably done by dividing the slice-like ROI into sub-regions, such that each sub-region includes nuclei corresponding to a specific resonance frequency band. Different subregions are generally located at different radial distances from the probe edge. Therefore, in order to excite a specific sub-region, the carrier frequency of each transmission pulse burst is determined according to the specific resonance frequency of the nuclei residing in that sub-region. By stepping the carrier frequency from one pulse burst to the next, the entire ROI can be scanned. Using extremely short pulses of sufficient power allows each sub-region to be substantially wide, meaning that a single pulse can excite nuclei in a substantially wide radial distance. Therefore, an image of the ROI can be acquired by a few pulse bursts.
The NMR signal received in each spin echo can be readily transformed into a one-dimensional density profile (neglecting NMR relaxation and diffusion effects) of the nuclei in the excited sub-region, along a radial vector. This is because each echo is acquired under a static (inherent) radial field gradient
For obtaining two-dimensional images, an angular gradient coil can be used, which may be of any known kind, for example such as disclosed in co-pending application assigned to the assignee of the present application.
The main conceptual novelty of this invention is that by combination of an external high-intensity static field and an internally produced local field gradient (by means of an internal field deforming imaging probe), a region is created in the local surroundings of the probe, which is characterized by substantially high field gradients, which allow high-resolution images of this region to be obtained. The use of internally produced gradients eliminates the disadvantages of external gradient coils with their acoustic noise and nerve irritation. Additionally, the substantial field gradient values created by the probe, allow homogeneity requirements from the external field source to be greatly reduced. If, in addition, the receiver coil is integrated in the probe, then a substantially high SNR is achievable, again allowing substantially high-resolution images to be obtained. In addition, the imaged FOV is substantially extensive, relatively to the dimensions of the imaging probe, and to dimensions of FOV created by the prior art totally autonomous probe.
Furthermore, the invented concept allows the production of local high-resolution images using relatively low-cost setup (probe, imaging console and a source of external static field, which for the purposes of the present invention may be substantially less homogeneous than as required for the conventional MRI techniques).